MPGD2015 will be set by the publisherDOI: will be set by the publisherc© Owned by the authors, published by EDP Sciences, 2016

Development of Gating Foils To Inhibit Ion Feedback Using FPC ProductionTechniques

D. Arai,1,a K. Ikematsu,2A. Sugiyama,2M. Iwamura,1A. Koto,1K. Katsuki,1K. Fujii,3T. Matsuda,3

1Fujikura Ltd., 1440 Mutsuzaki, Chiba 285-8550, Japan2Saga U., Saga 840-8502, Japan3High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba 305-0801, Japan

Abstract. Positive ion feedback from a gas amplification device to the drift region of the Time ProjectionChamber for the ILC can deteriorate the position resolution. In order to inhibit the feedback ions, MPGD-basedgating foils having good electron transmission have been developed to be used instead of the conventional wiregate. The gating foil needs to control the electric field locally in opening or closing the gate. The gating foil witha GEM (gas electron multiplier)-like structure has larger holes and smaller thickness than standard GEMs forgas amplification. It is known that the foil transmits over 80 % of electrons and blocks ions almost completely.We have developed the gating foils using flexible printed circuit (FPC) production techniques including animproved single-mask process. In this paper, we report on the production technique of 335 µm pitch, 12.5 µmthick gating foil with 80 % transmittance of electrons in ILC conditions.

1 Introduction

Time Projection Chamber (TPC) with a Micro PatternGas Detector (MPGD) readout has been proposed as acentral tracker of International Large Detector (ILD) forthe International Linear Collider (ILC) [1]. GEM is oneof the MPGD options to be used of the ILC-TPC gas-amplification device [2]. F. Sauli proposed the conceptof GEM as a gating device [3]. Despite ion-blocking po-tential of GEM, a small portion of back-flow ions are ac-cumulated during beam collisions. Ion discs are formed inthe drift volume of TPC due to a very slow drift velocityof the ions. As the primary electrons pass through the iondiscs on the way to the amplification module, the positionresolution of TPC might be deteriorated [4]. Using a gat-ing device to stop the ion feedback from the amplificationmodule is effective at preventing this problem thanks tothe ILC beam-train structure. This device can be used tostop ions back-flow simply by reversing the electric fieldin the holes of the gating foil and opening or closing thegate. Achieving a high electron transmission in a largemagnetic field for an open GEM gate demands that cer-tain conditions to be fulfilled. Simulation studies showedthat only a large aperture (large hole diameter and narrowrim) and very thin foil could meet our requirements [5].Conventional wire gating was another possibility. How-ever, the device needs a mechanical structure to maintainwire tension, and the electron trajectories may be distorteddue to E x B effects near wire. Nevertheless, the require-ment of an optical aperture larger than 80 % is challengingeven for FPC manufacturers who specialize in processing

aEmail : daisuke.arai@jp.fujikura.com

of fine electrodes. This report describes the requirementsfor gating foil for ILC-TPC in section 2, R&D of produc-tion methods of the gating foil in section 3, and finallylimitations of application of the process in section 4.

2 Requirements for gating foil

As ILC-TPC is operated in a 3.5 T magnetic field, the dif-fusion of the electrons can be reduced by a gas mixturewith a high ωτ. The ILC-TPC requires a good positionresolution of less than 100 µm, which can be obtained evenfor a long drift length of up to 2.3 m. The performance ofelectron transmission through gating foil is also affectedby the high magnetic field. It is understood that the rateis closely related to the optical aperture of the gate struc-ture [6]. The prototype gating foil has been studied in a 1T magnetic field and shown an electron transmission rategreater than 80 %. The results from the study above andthe simulation study showed that 80 % transmission wasobtained with proper potential setting of the gating foil ina 3.5 T B field. We have extended the area of the gatingfoil from the 100 mm x 100 mm for the test module tothe 170 mm x 220 mm for the ILC-TPC module keepingthe same specifications as the prototype. The 80 % opti-cal aperture and 12.5 µm insulator thickness are needed tosatisfy over 80 % electron transmission. The gating foilshould have a hole diameter of less than 300 µm to keepelectron position information. The details of requirementsfor a gating foil and an amplification GEM for ILC-TPCare summarized in Table 1.

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Table 1. Requirement specs of gating foil and amplificationGEM for ILC-TPC

Item Gating foil Amplification GEMOptical aperture ratio ≥ 80 % 22.7 %

Hole size ≤ 300 µm 70 µmHole pitch ≤ 335 µm 140 µm

Rim width (Hole pitch - Hole size) ≤ 35 µm 70 µmInsulator thickness ≤ 12.5 µm 50 µm or 100 µm

Foil size 170 mm x 220 mm 170 mm x 220 mm

3 R&D of gating foil productionMany different processes, including single mask, doublemask and laser drilling, have been applied to make am-plification GEMs [7]. Each processing method uses pho-tolithography and insulator removing techniques. Thoseare also used in FPC production processes such as a circuitformation and a though-hole formation process. We havedeveloped prototypes of the gating foil considering suit-able conditions for each process and improvements. Thefollowing subsections briefly describe the process we haveused in developing the gating foil.

3.1 Laser drilling process

Both the copper and the polyimide layer can be processedat the same time using Gaussian beam of a UV-YAG laserwith a 335 nm wavelength and about a 30 µm beam size .We moved the laser beam along a circle to make 300 µmholes (see Fig.1). The gating foil has a hole pitch of 330µm, a polyimide thickness of 25 µm, and a rim width onthe front (back) side of 14 µm (28 µm) on the 10 mm x 10mm area of the test gate. The optical aperture ratio is 75%. We verified that the same pattern was also able to beformed on 12.5 µm thick polyimide foil. The copper elec-trodes on the front side are apt to come off the polyimidesheet if the rim is narrower than 10 µm. The acceptableminimum rim width is therefore about 25 µm, which al-lows a maximum optical aperture of 77 % with this circlehole structure. The honeycomb structure with the samerim width and pitch can provide an 85 % optical aperturein principle, however, the laser machine cannot handle dif-ferent shapes other than circles because it is optimized forFPC production.

Figure 1. Gating foil produced by laser drilling process. (302 µmhole diameter, 330 µm hole pitch and 25 µm polyimide thicknesson the 10 mm x 10 mm area)

3.2 Double mask process

It is necessary to produce electrode patterns on both sidesof gating foil in a double mask process. The insulator is re-moved by a chemical process or laser etching. In this pro-cess, the copper patterns work as etching masks. The ac-curacy of the photomask alignment on both sides is morethan 10 µm for large area processing. Creating a 30 µm rimwith that process is therefore unacceptable. We concludethat the double mask process is not suitable for producingthe gating foil.

3.3 Single mask process

Although mask alignment in a single mask process is notnecessary, how to make an electrode pattern on the oppo-site side is another issue. We have approached this prob-lem by two methods.

3.3.1 Single mask process with Ni plating

We considered using Ni plating to protect the copper elec-trode formed on the front side when the copper on theback side is etched later. After the circuit production onthe front side, a defocused beam of a UV-YAG laser isirradiated to remove only the polyimide insulator wherethe copper electrode functions as a mask to stop the laserbeam. Laser etching enables the creation of less tapered,uniform-quality holes over a wide area than holes createdby chemical etching. A UV-YAG laser also seems to causeless damage on the surface of polyimide than a thermalprocess by a CO2 laser.

Figure 2. Cross-section view of the gating foil produced by sin-gle mask process with Ni plating

Figure 2 shows the cross-section of the gating foil pro-duced by a single mask process with Ni plating. The gat-ing foil has a polyimide thickness of 12.5 µm and a rimwidth on the back side of 36 µm. A gap was found be-tween the electrodes (Ni and Copper) and the polyimide.The gap was probably caused by the entry of the etchingliquid during copper etching on the back side.

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3.3.2 Single mask process without Ni plating

We have invented a new single mask process, which al-lows the copper on the front side to be etched without pro-tecting the copper by Ni. We prepare the sheet having athicker copper on the front side than the back side so thatenough copper is left on the front side when the copperin the hole area of the back side is etched away. Afterthe honeycomb structure circuit on the thick copper side isformed, the polyimide is removed by a defocused beam ofa UV-YAG laser. In this process, the thick copper works asa etching mask. Finally, we etched the copper from bothsides, where the etching speed in the hole area was twotimes faster than other areas, and obtained proper patternsof the electrodes on the both sides. Figure 3 shows theschematic view of etching for the back-side copper.

Figure 3. Schematic view of etching of back side copper (Thecopper of hole area of back sides is etched from both sides, sothe etching speed is two times faster.)

Figure 4 and 5 show the gating foil produced by thesingle mask process without Ni plating. The gating foil hasa hole diameter of 304 µm on the back side, a hole pitchof 335 µm, a rim width of 27 µm (31 µm) on the front(back) side and an insulator thickness of 12.5 µm on the100 mm x 100 mm area. The optical aperture ratio is 82 %.We have produced the gating foil successfully. ILC-TPCJapan group have measured the electron transmission ofthe gating foil, and verified that the electron transmissionwas over 80 % under the magnetic field 0 T and 1 T [6].

Figure 4. Micrographs of the gating foil (Front side) producedby single mask process without Ni plating

Figure 5. Micrographs of the gating foil (Back side) producedby single mask process without Ni plating

4 Limitations of gating foil processing

We have established a stable production process for thegating foil of the 100 mm x 100 mm test area. However,we still have to know the maximum area for which thismethod is applicable. In addition, we need to know theminimum limits of the rim width and the hole diameter(keeping the same optical aperture) for further production.

4.1 Available processing area

We have to avoid circuit breaks on the front side that leadto electrode breaks in the post etching process. The area towhich this process is applicable relies on the probability ofa circuit break due to contamination during photolithogra-phy. To improve the production environment and optimizethe photo-resist, the gating foil with a 30 µm electrodewidth has been successfully produced over the 170 mmx 220 mm area of the TPC module.

4.2 Minimum rim width

Figure 6 shows the cross-section of the gating foil having a7 µm (15 µm) rim width on the front (back) side. The nar-row copper of less than 7 µm peeled off the polyimide. Weconclude the minimum rim width by this process shouldbe 15 µm.

4.3 Minimum hole size

We have tried to make gating foil with a hole diameterof 100 µm and a rim width on the back side of 20 µmto minimize the hole size. The shape of the electrodes onthis foil was not a honeycomb form as shown in Fig.7. Thecorner of that small hexagonal holes could not be formedusing a common chemical process for circuit formation. Adiameter of 150 µm seems to be the minimum limit of theprocess.

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Figure 6. Cross-section view of the gating foil which has a rimwidth on the front (back) side of 7 µm (15 µm)

Figure 7. Micrographs of gating foil which has a hole diameteron the front side of 100 µm

5 Conclusion

We have developed the gating foil that has a GEM-likestructure and can provide an 82 % optical aperture overthe 100 mm x 100 mm test area and the 170 mm x 230mm module area. This will be a viable gating device toblock ion-backflow for ILC-TPC. The gating foil mountedon the ILC-TPC prototype module has been produced andthe electron transmission will be measured soon.

References

[1] The ILC Technical Design Report (TDR) : Volume 4 -Detectors,http://www.linearcollider.org/ILC/Publications/Technical-Design-Report.

[2] R. Yonamine et al. JINST. 7, C06011 (2012)[3] F. Sauli et al. NIM A560, 269 (2006)[4] D. Arai, Master thesis (2012), Tokyo University of

Agriculture and Technology (TUAT)[5] P. Gros et al.JINST 8, C11023 (2013)[6] K. Ikematsu on behalf of the LCTPC-Japan Collab-

oration, IEEE Nuclear Science Symposium and Med-ical Imaging Conference (NSS/MIC) 2014 ConferenceRecord (N44-7).

[7] T. Tamagawa et al. NIM A560, 418-424 (2006)